1. Field of the Invention
The invention relates in general to a microwave cavity, and more particularly to a cylindrical resonant cavity that can be used to measure the dielectric characteristics of various materials.
2. Description of the Related Art
The applications of microwave technology have attracted the attention of researchers and industry and these applications include: material characteristics measurement, nondestructive detecting, communication, radar, medical science, biochemistry and agriculture. Since the related research requires knowing precisely the material""s dielectric characteristics, the researchers have devoted themselves to the exploration of dielectric material. This makes the development of microwave technology more prosperous.
In the electronics industry, the improvements of the microwave engineering make high frequency communication technology more advanced, from the early days of satellite transmission to the personal portable communication devices. The process of high frequency circuit fabrication is to form the layout on the circuit board first, and after the completion of the layout, the related components are assembled to complete the whole circuit. It is important to realize that since the circuit board is a kind of dielectric material and the electric characteristics are decided by the individual parameters of the dielectric material. Therefore, one must master the dielectric characteristics of the circuit board before starting the circuit design. Thus, the parameters such as permittivity, loss tangent and the Q factor are essential information to make sure the quality of the circuit board is as expected. There are many measuring techniques available for measuring the parameters of dielectric materials, for example, wave-guide method, transmission method, microstrip line method, cavity perturbation method (CPM) and quasi-optical resonator method. Among these methods, CPM and quasi-optical resonator method produce the lowest loss in measuring the loss tangent. The paragraphs below contain the explanation about the CPM.
The CPM involves placing a diminutive sample into the cavity to cause perturbation and change the resonant frequency of the cavity and its Q factor, so the dielectric characteristics of the sample can be calculated from the quantity of those changes. Since CPM is particularly suitable for measuring the dielectric materials having a high Q factor, it is favored by most researchers.
FIG. 1 illustrates the cross-sectional view of the cavity and the diminutive sample during the CPM. It can be seen from FIG. 1 that a diminutive sample 130 is placed into a cavity 100 which is then excited. The dielectric characteristics of the diminutive sample 130 can be calculated from the volumes of the sample 130 and the cavity 100, and the changes in resonant frequency and the Q factors, which can be derived from comparing the measurements before and after the insertion of the sample 130.
FIG. 2A presents a cylindrical cavity with one end being a top end 200a and the other a bottom end 200b with both ends sealed to form a closed space between the top end 200a and the bottom end 200b. FIG. 2B is a cross-sectional view of the cavity 200 taken along line 2Bxe2x80x942B in FIG. 2A.
Referring to FIG. 2B, one must excite the cavity 200 and measure the resonant frequency and the Q factor of the cavity 200 before performing the CPM. Then a diminutive sample (not illustrated in FIG. 2B) will be placed into the cavity 200 in a manner that is shown in FIG. 1; after the insertion of the sample, the cavity 200 will be excited again in order to measure the changed resonant frequency and the Q factor.
According to theory, the resonant frequency of the cavity in TM012 mode is:             f      012        =                  c                  2          ⁢                      xe2x80x83                    ⁢          π                    ⁢                                                  (                              2.405                a                            )                        2                    +                                    (                                                2                  ⁢                                      xe2x80x83                                    ⁢                  π                                l                            )                        2                                          c      =              3        xc3x97                  10                      8            ⁢                          xe2x80x83                                      ⁢        m        ⁢                  /                ⁢        s              ⁢          
        ⁢    a    =          radius      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      inner      ⁢              xe2x80x83            ⁢      wall      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      activity            l    =          length      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      inner      ⁢              xe2x80x83            ⁢      wall      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      activity      
If a=1.85 cm, and l=7.7 cm, then the resonant frequency will be
xe2x80x83f012=7.33 GHz.
After the insertion of the diminutive sample, the resonant frequency and the Q factor will change and the dielectric characteristics of the diminutive sample can be derived from these changes. It is important to note that the essential condition of the CPM is that the Q factor of the cavity must be higher than that of the diminutive sample; otherwise, the accuracy of the measurement will be affected.
Traditionally, the high Q factor cavity is in TM010 mode and is excited by transmission. The Q factor of this kind is under 5000 due to the restraint of the cavity structure. In other words, when the Q factor of the measured dielectric material is greater than 5000, the resulting measurements will not be accurate; thus, it will be meaningless to carry out the CPM.
It is therefore an object of the invention to provide a loop coupled microwave cavity apparatus, which can be excited by TM012 mode microwave signal, in order to provide a higher Q factor to the cavity for measuring the dielectric characteristics of the material having high Q factor.
The invention achieves the above object by providing a new loop coupled microwave cavity apparatus and its features are described in the following paragraphs.
The loop coupled microwave cavity apparatus includes a cavity and a loop coupler. In the process of making the cavity, a copper pillar is drilled to form a hollow, and a step base is made and connected tightly to the hollow in order to form the main body of the cavity apparatus. Then, a mushroom-shaped lock hole is made on the top of the cavity apparatus by drilling. The lock hole is to be used for the insertion of the loop coupler. The loop coupler has a receiving end and an excitation portion, wherein the receiving end is connected to an outside circuit in order to receive a microwave signal from the outside circuit while the excitation portion is connected to the inner wall of the cavity in order to excite the cavity. In practice, one can use an SMA connector having a long pin as the loop coupler. The connecting part can be used as a receiving end and the tail of the long pin is bent to form the excitation portion. The long pin of the SMA connector is placed into the cavity through the lock hole while the end of the tail of the long pin is connected to the inner wall of the cavity; then a microwave signal in TM012 mode can be fed to excite the cavity. On the other hand, the long pin and the lock hole form a coaxial structure, which can be viewed as a quarter-wavelength transformer. Therefore, the SMA connector serves not only as a loop coupler but also as an impedance transformer to increase the Q factor of the cavity. Furthermore, one side of the cavity can be drilled to form a side hole through which the diminutive sample can be placed into the cavity to perform the CPM.